The present invention generally relates to radio communication devices and systems and specifically to mixers in radio communications devices.
Balanced mixers are used in radio receivers for transforming incoming radio frequency (RF), containing numerous frequency signals, to an intermediate frequency (IF). A mixer is a non-linear device that receives two signals, one from an (RF) antenna that is amplified by a Low Noise Amplifier (LNA), and the other from an internal local oscillator (Lo), and produces a third output signal. The resulting output signal can be demodulated using a variety of demodulation schemes and presented to a user. Since the output signal is a host of sum and difference terms, the mixer can be used to step up the frequency of the input RF signal.
Balanced mixers have been used extensively, together with other configurations, in various radio systems.
Monolithic double balanced mixers are generally based on structures known as Gilbert cell, first described by Gilbert in 1969. Although the Gilbert type mixer has excellent carrier suppression and low second order distortion, it suffers from a high noise figure and intermodulation distortion.
FIG. 1 presents a typical structure for a Gilbert cell mixer known in the prior art. The mixer core includes four transistors Q1, Q2, Q3, and Q4 that are coupled to differential local oscillator inputs Lo+ and Loxe2x88x92. The RF signal is present at the inputs RF+ and RFxe2x88x92 as a differential (balanced) signal and is coupled to the mixer core using transistors Q5 and Q6. The RF signal is amplified and converted from voltage to current by transistor pair Q5 and Q6 and is switched by the mixer core at the rate of the local oscillator. The switched current produces an intermediate frequency signal IF, at the outputs Outxe2x88x92 and Out+, having a frequency described by the following equation:
Frequency (IF)=Frequency (Lo)xe2x88x92Frequency (RF)
The resistors Rc1 and Rc2 are used for converting current information into voltage information. A degeneration resistor Re couples the emitters of transistors Q5 and Q6 to the current sources Io1 and Io2. Degeneration resistor Re improves the input third order intercept point (IIP3) for the incoming RF signals. However, increasing the value of degeneration resistor Re will degrade the noise figure and the mixer gain. Input impedance, provided by the input differential transistor pair Q5 and Q6 with emitter degeneration, will be high due to series feedback, and will require additional circuitry for input impedance matching.
In view of the foregoing, it is an object of the present invention to provide a balanced mixer which obviates and mitigates from the disadvantages of the prior art and improves the overall performance of the mixer circuit input third order intercept point (IIP3), noise figure, and input impedance matching ability.
According to the present invention, there is provided a balanced mixer comprising: a first differential input port to provide input radio frequency to the balanced mixer; a second differential input port to provide local oscillator frequency to the balanced mixer; a transconductance amplifier means connected to the first input port; a splitter circuit means connected to the amplifier means for splitting the input radio frequency; a mixer means connected to the splitter means and to the second input port for receiving the split input radio frequency and the local oscillator frequency; a feedback means connected between the splitter means and the first input port to provide feedback current to the first input port; and an output port connected to the mixer means.
One advantage of the present invention is that the feedback means improves the overall performance of the mixer circuit input third order intercept point (IIP3), noise figure, and input impedance matching ability.
Other advantages, objects and features of the present invention will be readily apparent to those skilled in the art from a review of the following detailed descriptions of a preferred embodiment in conjunction with the accompanying drawings and claims.